A truck breaks down on the Gardiner just before 8 a.m. on Friday as some 4,000 cars per hour try to enter the city's core. A police cruiser arrives within 10 minutes, but a heavy-haul tow truck cannot get there quickly. In the hour it takes to move the truck off the road, traffic has backed up to the edge of the city.

Two days earlier, also during the morning rush hour, a person is hurt on the subway track at Queen's Park station, forcing the shutdown of part of the downtown Yonge line. It takes 35 minutes for emergency workers to clear the scene and another 20 for the backlog of passengers to clear. Some 70,000 TTC passengers are directly affected, but the impact ripples across the network.

Neither incident was remarkable, yet both caused thousands of people to be late, and laid bare the fragility of Toronto's transportation systems. In a city with some of the longest commute times on the continent, it does not take much to make a bad situation worse.

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"We have jeopardized the economic competitiveness of this region and that's the price we're paying now," said Murtaza Haider, director of the Institute of Housing and Mobility at Ryerson University. "We are not at capacity, we have been beyond capacity … for the last 20 years."

Toronto's major highways and transit lines have a common challenge: Amid booming urban growth, they struggle to absorb the fallout of everyday incidents. And solutions are not easy.

The partial shutdown of the subway on Wednesday overloaded the Bloor-Yonge interchange – at times, trains on the east-west line were bypassing the station, and walk-in passengers were being prevented from entering – and it took around an hour for normal travel to resume.

"Everybody who's riding the system is affected because it does have a knock-on effect," TTC spokesman Brad Ross said.

That same ripple effect was felt on the Gardiner on Friday morning, when a large truck stalled near Jameson Avenue. Drivers on the key commuter artery said the incident backed up traffic and shifted the point at which stop-and-go congestion begins several kilometres farther west. This jam-up also made it harder for the tow-truck to get to the breakdown, where police sat waiting with lights flashing.

"A police car's not going to push a heavy truck," Constable Clint Stibbe said. "Because the tow truck doesn't have any emergency equipment that would allow priority to get down the road, it took the tow truck from 8:05 am until 9:15 am to arrive on scene."

The broken-down truck was moved moments later, but the damage to traffic had been done.

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Traffic engineers have identified the so-called shockwave effect, which sends the impact of a slowdown farther back the road. A Japanese study showed that this can occur even without an apparent trigger. If sparked by an incident, especially one that closes a lane of traffic, the effects can be profound.

Joey Gagne, president of Abrams Towing Services, said the city could consider paying to have heavy tow-trucks positioned at key points on the highways. He said the cost – about $200,000 per year for each truck and driver, he estimated – could be worth it when considering the economic impact of traffic back-ups.

Studies put the annual cost of congestion in the Toronto region at upwards of $6-billion.

Mr. Haider said the city has to think about expanding the roads. He acknowledged long-standing research showing that the new roads would fill up, but said they still would have value by being able to carry more vehicles.

The sort of problems the TTC faced this week are also difficult to tackle. The system carries about 10 million people a week, and Bloor-Yonge is a choke point at the best of times.

Mr. Ross said the downtown part of the subway system does not have the physical space to consider double-tracking it, which in any event would be hugely expensive. More crossovers (points where trains can switch to the other track and turn back) could help minimize the amount of the system that needs to be shut during a problem, but would be costly. New crossovers have been built around College and King stations, but they will not be operational for a few years.

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Although a relief line would could help the TTC divert some passengers in such incidents, Mr. Ross said that feature should not be considered a selling point of such an expansion. Platform-edge doors, which would keep unauthorized people off the track and reduce the many delays due to track fires, would cost hundreds of millions of dollars, according to TTC figures.

In some ways, though, both of these problems were simply bad luck.

"At 8:30 in the morning, when this happened, that is really at the height, or the peak if you will, of the peak morning rush," Mr. Ross said. "At any other time, the knock-off effect of crowding wouldn't have been as noticeable because you don't have as many people riding."